An exhaustive description of the methods for obtaining glycidol may be found in Axel Kleemann & Rudolf M. Wagner (1981). Glycidol: Properties, reactions, applications. Heidelberg; Basel; New York: A. Hüthig; ISBN: 3-7785-0709-5. Glycidol can thus be obtained by epoxidation of allylic alcohol with hydrogen peroxide, hyperperoxides or percarboxylic acids; by dehydrohalogenation of monohalohydrins of glycerol using hydroxides of alkaline metals or alkaline earth metals; from acrolein by epoxidation and subsequent catalytic hydrogenation of the glycidaldehyde obtained using sodium chromate in tetrahydrofuran or metallic hydride reducing agents such as potassium borohydride in ethanol-water or lithium hydride and aluminum in absolute ether.
All these processes have the common drawback of using highly toxic raw materials. In addition, the epoxidation of allylic alcohol, carried out industrially by Degussa AG (at present Evonik Industries) using hydrogen peroxide and a catalyst based on NaHWO4 is an industrially inconvenient process, since on the one hand, in the reaction step a reaction system is used formed by three reactors connected in series plus a fourth to ensure the consumption of the hydrogen peroxide traces, and on the other hand in the separation step, an evaporator and five distillation columns are used since the reaction mixture is formed by allylic alcohol without reaction, glycidol, the catalyst, water and small amounts of impurities such as glycerol, acrolein, beta-hydroxypropionaldehyde, glycerin-1-allyl ether and beta-allyloxypropionaldehyde. The dehydrohalogenation of monohalohydrins of glycerol, since it is not a catalytic process, uses stoichiometric or greater quantities of hydroxides of alkaline metals or alkaline earth metals causing the generation of enormous quantities of the corresponding halides therefore it is not economically profitable if said halides are not used in an adjacent production plant for electrolytically regenerating the hydroxides, a process which consumes a large quantity of energy. The epoxidation of the acrolein followed by the hydrogenation of the intermediate glycidaldehyde has the drawback that low outputs are obtained if sodium chromite is used as the catalyst or that very expensive reducing agents are used such as metal hydrides which negate the industrial viability of the process.
Given the drawbacks of said methods, alternative methods have been sought for producing glycidol, such as the method which proceeds using glycerol carbonate, a non-toxic compound which can be easily produced from glycerol by reacting both with urea (S. Fujita, Y. Yamanishi, M. Arai, ournal of Catalysis 2013, 297, 137-141) and with dimethyl carbonate (J. R. Ochoa-Gómez, O-Jiménez-Aberasturi, B. Maestro-Madurga, A. Pesquera-Rodríguez, C. Ramírez-López, L. Lorenzo-lbarreta, J. Torrecilla-Soria, M. C. Villarán-Velasco. Applied Catalysis: General A 2009, 366, 315-324).
Obtaining glycidol by decarboxylating glycerol carbonate can be carried out thermally in a vacuum (to continuously separate glycidol and avoid its polimerization) without the use of a catalyst as emerges in examples 4 and 5 (not part of the invention) of this specification and is described in the U.S. Pat. No. 7,888,517 B2. The method is especially advantageous with respect to the other methods for obtaining glycidol described above, since pure glycidol is obtained in only one step by combining the decarboxylation of the glycerol carbonate with the separation by evaporation in a vacuum of the glycidol formed. However, obtaining industrially significant outputs, for example of 60% or more requires work at high temperatures, greater than 175° C. At temperatures of 140-160° C., the outputs do not exceed 45%, reducing in accordance with the reduction of the temperature, such that at 140° C. they are less than 2% and there is no reaction at temperatures equal to or less than 130° C. (see the examples 1-5 (not part of the invention) of this specification).
Therefore, the methods that have been available up to now for obtaining glycidol from glycerol carbonate are carried out in a vacuum and at temperatures greater than 150° C., more frequently at temperatures greater than 175° C. which involves high power consumption. Thus, for example, document U.S. Pat. No. 7,888,517 B2 describes a method for obtaining glycidol from glycerol carbonate in a vacuum without using a catalyst or using as such a neutral salt of an alkaline metal such as sodium sulfate or chloride or an alkaline earth metal. Such a method leads to low outputs of 39-45% when it is carried out at temperatures between 155° C. and 160° C. (examples 2 to 4 of said invention) and requires continuous work at a temperature of 200° C. (drop by drop addition of glycerol carbonate) to obtain high outputs (66% to 79%, examples 4 and 5 of said invention). It is therefore a method that is very expensive in terms of energy costs.
Document U.S. Pat. No. 2,856,413 describes a method for obtaining glycidol from glycerol carbonate using such basic catalysts as phosphates, pyrophosphates, chlorides, bromides, acetates, carbonates and bicarbonates of alkaline metals and alkaline earth metals. As patented in the examples, the obtainment of outputs greater than 72% requires work in a vacuum and at temperatures greater than 195° C. Therefore this method is also very expensive in terms of energy costs.
Document U.S. Pat. No. 5,359,094 describes a method for obtaining glycerol carbonate by carboxylation of glycerol. The synthesis of glycidol is not claimed, however, in its specification (page 2, lines 25-40) its synthesis by decarboxylation of glycerol carbonate is mentioned using a most preferred temperature range of 210 to 275° C., as is exemplified in its examples 3 and 4 and using as catalysts salts of alkaline metals or alkaline earth metals, such as halides, phosphates, monohydrogen phosphates, pyrophosphates, sulfates, borates, acetates, carbonates and bicarbonates. The method is very expensive in terms of energy costs.
Document U.S. Pat. No. 6,316,641 describes a method for obtaining glycidol from organic cyclic carbonates, in particular glycerol carbonate, in a vacuum in the presence of a polyol as a solvent and a solid catalyst which comprises a type A zeolite or γ-alumina. However, temperatures greater than 165° C. are claimed, temperatures greater than 180° C. being necessary for obtaining outputs greater than 70%, therefore this method is also very expensive in terms of energy costs. In addition, the active hydrogens of the terminal hydroxy groups of the polyol may be activated, producing the ring-opening polymerization both of the starting glycerol carbonate and the formed glycidol.
Document U.S. Pat. No. 7,868,192 B1 describes a method for obtaining glycidol from glycerol carbonate in a vacuum in the presence of solvents which do not have active hydrogens such as liquid parafins and/or polyalkylene glycol dimethyl ether, using neutral salts of alkaline metals and alkaline earth metals as catalysts. There is no temperature range claimed but the one used in the examples is between 180° C. (output 60%, in a vacuum, example 5) and 250° C. (example 6: environmental pressure, nitrogen bubbling, output 70%). This method is also very expensive in terms of energy costs.
Document US 2014/0135512 A1 describes a method for obtaining glycidol from glycerol carbonate using as catalysts ionic liquids derived from methyl imidazolium in which the anion of the ionic liquids has a basicity in the range of 0.60 to 0.80 based on the Kamlet-Taft parameter. The ionic liquids can be used alone or in combination with a Lewis acid metallic salt such as Zn(NO3)2, ZnCl2, SnCl4, MgCl2, AlCL3 and its mixtures. This method has the drawbacks of high cost of the ionic liquids and the high working temperature between 165° C. and 175° C. for obtaining outputs of industrial interest such as emerge in Table 6 of said document. The output of the reaction is 0% at 140° C.
In addition, various methods have been described for obtaining glycidol at temperatures lower than 100° C., proceeding from glycerol and dimethyl carbonate, using superbase catalysts such as DBU and ionic liquids and quaternary ammonium hydroxide. The glycerol reacts with the dimethyl carbonate in the presence of the basic catalyst to give glycerol carbonate which decarboxylates to glycidol. See for example, S. K. Gade, M. K. Munshi, B. M. Chherawalla, V. H. Rane, A. A. Kelkar, Catal. Commun. 2012, 27 184-88; R. Bai, H. Zhang, F. Mei, S. Wang, T. Li, Y. Gu, G. Li, Green Chem. 2013, 15, 2929-2934; Y. T. Algoufi, U. G. Akpan, M. Asif, B. H. Hameed, Appl. Catal. A Gen. 2014, 487, 181-188, M. K. Munshi, S. M. Gade, V. H. Rane, A. A. Kelkar, RSC Adv. 2014, 4, 32127-32133 and Y. Zhou, F. Ouyang, Z.-B. Song, Z. Yang and D.-J. Tao, Catal. Commun. 2015, 66, 25-29. However, these methods lead to a complex mixture formed by the reagents without reacting, intermediate glycerol carbonate without reacting, glycidol as well as the catalysts and solvents used from which it is difficult to separate the glycidol given the ease with which it polymerizes. These methods are therefore not industrially viable.
Therefore, there is a need for an industrially viable method for producing glycidol.